ST1100 Airbox and Carburetor Jet Modifications


Better Running through Better Breathing


Jim Steinborn

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Even with some pointers, changing your carburetor jetting can be a trying experience. There were some times when I wasn't sure that it was going to turn out well at all. Fortunately, for the cost of a new airbox lid, this can all be undone if it doesn't work out to your satisfaction. In any event, it will probably take quite a bit of fiddling to get everything just so. I do feel, however, that the results justified the effort. In any event, please read through this article completely before deciding to do this yourself.


My partner, Bill Rogers, has a fairly early production 1991 model (ser# 100723, purchased in July, 1990). At the time of the ST purchase, he also owned a Yamaha V-Max and, still used to its claimed 145 hp, was somewhat disappointed in the power of the ST (I should mention, for those that are not familiar, that the V-Max has a device called 'V-Boost' which allows each cylinder to draw fuel mixture from two carburetors at 6,000+ rpm, causing the bike to literally explode forward. The ST, as we all know, has an unusually even power delivery, without much of a top-end 'rush' compared to bikes with peakier power bands).

To improve the ST, he took it to Love Cycles in Houston for their so-called 'Dyno Tune.' This procedure involves enlarging the airbox inlet, which allows the engine to breathe better. They spend a lot of time on the dynomometer with an exhaust gas analyzer to get the mixture set correctly across the entire powerband. The result was a claimed 17% horsepower increase accompanied by a substantial increase in intake noise, as well as a huge dent in the wallet. The increased intake noise is not annoying to others (like a loud pipe might be), but it can be kind of loud while grinding up long grades. On the other hand, some people think that the extra intake noise makes the ST sound better, because it masks the valve gear whine, especially when you have it 'whacked open.'

Comparing the 'Dyno Tuned' ST to the stock one was interesting. The modifications resulted in a stronger midrange as well as a much more impressive high-rpm performance. The stock ST seems to start running out of breath above 6,000 rpm. At high engine speeds, it appears that the airbox is just too restrictive to allow easy breathing (one of the reasons for the restrictive airbox from the factory has to do with reducing intake noise to meet EPA standards).

Carburetor Basics

The last thing I intend is to insult the intelligence of anyone reading this, but many of us aren't very conversant with the inner mysteries of constant vacuum (CV) carburetors. I think that the rest of the re-jetting procedure will be more interesting with an overview of CV basics. The October, 1995 issue of Motorcyclist magazine has a good article on this subject. Much of this information has been gleaned from this article and from the Honda Common Service Manual.

The carburetors on the ST1100 belong to the family of 'variable venturi' carburetors. The variable venturi continuously changes the diameter of the main bore in proportion to the volume of the incoming air. This gives a small diameter at idle and low speeds, so that the incoming air has sufficient velocity to draw fuel up from the float chamber. At large throttle openings, the bore is wide open, allowing unrestricted airflow to maximize power output at high engine speeds.

On the top of the carburetor, there is a vacuum chamber containing a diaphragm attached to a piston (the vacuum piston). The vacuum piston resides in a bore that is perpendicular to and intersects the main bore of the carburetor. At idle, the piston is pushed most of the way down by a spring, almost completely closing off the main bore. As the throttle butterfly valve is opened, the airflow in the main bore exerts a negative pressure on the lower section of the vacuum piston. At this point, air is drawn from the vacuum chamber through a hole in the bottom of the piston, overcoming the spring pressure and causing the piston to rise.

In the float chamber, the main jet is screwed into the bottom of a tube called the needle jet. The needle jet opens into the main bore of the carburetor and allows the fuel into the intake manifold by means of the negative pressure formed by the intake air rushing through the venturi. This is the very same principle witnessed by blowing across the top of a soda straw and drawing up the liquid.

The piston carries the 'jet needle' that fits into the needle jet (aren't you just loving this?). The jet needle is straight for about 1/3 of its length; the rest is tapered. At idle and low speeds, the piston is nearly all the way down, pushing the needle into the needle jet most of the way. In this position, the straight, large-diameter part of the needle (the root) fills up most of the space inside the needle jet tube, restricting the fuel flow to a narrow annular space around the needle. As the piston rises with increased engine speed, the needle is withdrawn from the jet. Because the needle is tapered, the annular space through which the fuel can travel increases, allowing more fuel to match the increased airflow.

From idle up to about 1/8 throttle (i.e. moderate speed cruising), the major source of fuel is the pilot (slow) circuit. This includes the pilot jet and the pilot screw adjustment. On the ST1100's carburetors, turning the pilot screw out results in a richer mixture at small throttle openings.

As you can see, there are a huge number of variables involved in proper fuel delivery:

  1. From zero to 1/8 throttle, the pilot circuit is the major contributor, influenced somewhat by the root diameter of the jet needle.
  2. In the lower midrange, the starting point of the needle taper is crucial.
  3. In the upper midrange, the degree (steepness) of the needle taper is significant.
  4. In the high speed range, the main jet is the biggest factor.
This doesn't even consider the tension of the vacuum piston spring or float height, but I didn't find it necessary to mess with any of that.

The Torque Monster

The ST1100's engine is engineered for a very flat and broad torque curve. This is done at the expense of ultimate top-end power. Case in point: with about the same engine displacement, the Kawasaki ZX-11 outputs 131 hp at 10,500 rpm at the rear wheel compared to the ST's 90 (or so) hp at 7,500 rpm. On the other hand, the ZX-11 only makes one more lb-ft of torque at 8,500 rpm than the ST's 77 lb-ft at 5,000 rpm. I'm not trying to imply that the Kawasaki is in any way deficient in the torque department, but its engine has been designed for maximum top-end power at the (relative) expense of torque.

One of the ways to engineer for a generous torque curve is to design in a high intake velocity. This is done on the ST by using long, narrow intake tracts. You no doubt have noticed the 'snorkles' in the airbox. I'm just guessing, but from the open end of the snorkle to the intake valve is probably 12 inches or more. Having the incoming air travel through these long and relatively small passages ensures that it will have a high velocity. The motion of this air column is being stopped and started by the opening and closing of the intake valves. When the valves close, the air column bounces off of them, only to rebound and start to rush toward the valves again. If the valves are timed correctly, they open just as the air column reaches them. This can improve the volumetric efficiency quite a bit. Unfortunately, it is only effective over a relatively narrow range of engine speeds, determined by the resonance of the intake tract. All of this helps to fatten up the midrange torque curve, but does not increase the top-end power much, if any at all. This is the same principal as the 'Ram Air' or 'Ram Jet' induction systems used on autos in the '70s.

One disadvantage of this approach is that it limits breathing at high engine speeds. There is only so much air that can be easily drawn down a narrow passage. I suspect that this is as much a factor in the low (8000 rpm) redline as is the longish stroke. You don't gain anything by spinning the engine faster if it can't breathe easily. As a result of all this, one cannot make huge increases in the horsepower of this engine without doing a major amount of work. One can, however, make more incremental changes. The modifications below aid the top-end breathing. When finished, the limiting factor is no longer the airbox inlet but the induction system itself.


You must be aware that any modification to the fuel delivery or emissions system of your motorcycle renders it illegal for operation on public roads. Being a law-abiding citizen, after these modifications, I naturally restrict my ST1100 to motocross and dirt flat tracks [grin]. In addition, modifying your motorcycle is inherently risky and you need to use your own judgment on the advisability of any portion of this procedure (no smoking, please). In short, if it seems like a bad idea, don't do it (apologies to Grant Norman for stealing his phrase).

Let's do it!

After my warranty expired, I decided that I could afford to make some more interesting modifications to my ST. I decided to try to duplicate, as best I could, the 'Dyno Tune.' As we now live in Fort Collins, Colorado, I called Paul Wunsch (owner of Love Cycles) for some information. He wasn't interested in telling me anything at all ("that information is proprietory [sic]"), so I was on my own. Here, now, is my attempt to replicate their results.

Needed Materials:


The airbox inlet on the ST is about 4" by 3/4" (I don't have one in front of me, so I'm trying to visualize). That's pretty small, even for a torque-monster engine with tiny (32mm) carburetors. That's why installing a less restrictive filter doesn't effect the mixture too much: the airbox inlet is the limiting factor.
  1. Bore 8 1-inch holes in the airbox lid, evenly spaced around the edge, outside of the perimeter of the filter. I smoothed the inside and outside edges of the new holes with sandpaper or a file (mostly just to satisfy my own obsessiveness).

  2. Remove the fairing pockets from both sides.

  3. Removing the airbox. (Note that California STs [and all STs with ABS, I believe] have extra emissions equipment that I don't know anything about, sorry :-( ) Remove the two screws holding the fuel petcock onto the airbox at about the 5:00 position. There are six (not four) screws holding the base of the airbox down to the carburetor air chamber. There are two nearly hidden screws at the 12:00 and 6:00 positions, underneath the 'snorkles.' There is a crankcase breather tube attached to the underside of the airbox base at the 3:00 position. It is retained by a spring clamp that is difficult to get to, but can be removed (after lifting up the base as far as possible) with needle nose pliers This is where the colorful language starts being useful :) The two tubes for the vacuum chamber balance (ambient) air are attached to a little box that can be snapped off the front of the airbox - don't lose the filter that lives in there. The inlet hose for the supplementary air injection system is removed from the underside of the front of the airbox base. Now you should be able to remove the airbox base with a bit of fiddling.


This is the time-consuming part.
  1. Unseating the carburetors. Loosen the band clamps that hold the carburetors onto the rubber connecting tubes by several turns. Grab hold of the bridge over the snorkles and/or grab around the edges of the air chamber and pull, first one side, then the other to pop the carburetor flanges out of the connecting tubes. This is difficult to do the first time, as these items are all stuck together pretty well. On occasion, I have run the engine a bit to warm up the rubber connecting tubes, allowing easier removal.

  2. Looking from the side while rotating the carb assembly up a bit, you will see a small recessed slotted screw in the bottom cover of each carb. This is the float chamber drain screw (as you can see, that little rectangular gray plug in front of your knees when you are seated on the bike is not for carburetor drain access, as is commonly thought). Put a catch basin under the bike (somewhere in front of the center stand, I think), and loosen the four screws and let the fuel drain completely. Re-tighten drain screws.

  3. Remove the fuel supply hose from the back of the carburetor assembly. Pull the assembly up and remove the four drain tubes from the underside of the carbs. Detach the choke and throttle cables from their anchor points. Remove the carburetor assembly from bike.

  4. Selecting jets. Apparently I didn't make my intentions very clear when I ordered the FACTORY jet kit, because the kit came with smaller main jets, apparently intended for rejetting for higher altitude (it is about 5,000 ft. here in Fort Collins). Therefore, I spent a lot of time running to the hardware and hobby stores for small drill bits. FWIW, I have heard that one should not drill out jets as this can lead to irregularities in the bore of the jets, causing uneven fuel delivery. To try to get around this, I held the drill bits in a pin vise in the drill press set to the highest speed. I inserted the bit slowly on the first pass and then ran it in and out a few more times in an attempt to hone the bore smooth. Another reason not to drill out the jets is that the bit sizes are limited: #55 gives 132 jets; #54 gives 139.5 jets; #53 gives 151 jets, none of which are quite the right size.

    According to the FACTORY instructions, you select the proper main jet by trying out different size jets and select the one that gives the highest top speed. Unfortunately, for most of us, trying to ascertain the top speed of the ST over and over is not very practical, but I did the best that I could. I tried: 132, 139.5 and 151 and ended up with 139.5. I would recommend starting with 135, possibly 138. More on this later.

  5. Changing the jets. Invert the carburetor assembly. Remove the four screws that hold one of the float chambers onto its carburetor body. Removing the float chamber, you will see two slotted brass screws with holes in their centers. The larger one is the main jet. The smaller one is the pilot (slow) jet. Using a screwdriver that fits very precisely, remove the main jet and replace it with your new jet. Now would be a good time to clean out the residue that has collected in the bottom of the float chamber. Replace the float chamber cover, ensuring that the o-ring gasket is arranged correctly. Reinstall the four screws. Repeat for the other three carburetors.

  6. Selecting needles. My jet kit came with 3 sets of needles, all varying in degree of taper. After interminable experimentation (consuming most of the three weeks I spent on the job), I selected the ones with the steepest taper (needle # 0972f-66q). The top of the needle has 5 grooves in it. These receive a spring metal 'e-clip' (supplied). The grooves allow you to adjust the height of the needle: the higher the needle, the sooner the taper takes effect. The grooves are numbered from the top, so #1 is the lowest position and #5 holds the needle up the highest. I settled on #3. More on this later, as well.

  7. Changing the needles. On the top of the carburetor, you will find a round, shiny cover. This is the vacuum chamber cover. Remove the four screws holding it on. These screws are sometimes very tight. My FACTORY kit came with replacement recessed hex head screws to replace these; I recommend using them. When removing the cover, keep an eye on the spring: it is about 3/4" in diameter and 5" or 6" long. Pull the spring out of the piston, which is attached to the black rubber vacuum diaphragm. Stick two fingers into the piston and spread them apart, pressing against the inside walls of the piston. Withdraw the piston from its bore. The edge of the diaphragm may be stuck down: pull it up gently.

    Down in the bottom of the piston you will see a hex head attached to a milky white plastic disk. With an appropriate socket (about 8 or 10mm, I think), gently turn the disk about 1/4 turn counter-clockwise, until it stops. Turn the piston over, covering the open end with the palm of your hand and push the needle into the piston, being careful not to lose any parts. As I recall, there is a spring pressed into the back of the white disk and a small flat washer in the base of the piston cavity You will notice that the stock needles have fixed heads and don't allow any adjustment short of shimming them up.

    Install the e-clip into the appropriate groove in the new needle, drop it through the hole in the bottom of the piston and reinstall the white retaining disk. When I did this, I noticed that the needle sometimes stuck out of the piston at an angle. I readjusted things until it was as straight as possible. Put the piston/diaphragm back into its bore, aligning the tab in the diaphragm edge with the groove in the vacuum chamber body. Reinstall the spring and attach the cover with the new screws (if supplied). Repeat for the other three. Reinstall the carburetor assembly onto the engine, pushing the carb flanges back into the connecting tubes and reversing the procedure for step 6. Make sure that the connecting tube band clamps are nice and snug (if there are any leaks there, all bets are off). Re-attach the air cleaner base and install the K&N filter. Screw down the air cleaner lid.

Tests and Adjustments

The FACTORY kit came with pretty good instructions regarding adjustments. Here is a brief summary.

Hopefully, the engine will start :^) Do not warm up the engine! Lean problems improve and rich problems get worse as the engine warms up, so starting the test with a cold engine can be beneficial. Starting cold, you can watch the progression of symptoms up to operating temperature. The engine does need to be at operating temperature, however, to be sure that a lean or rich problem definitely exists.

  1. Pilot circuit (zero to 1/8 throttle)-

  2. Full Throttle - lower midrange

    Adjust the needle clip so that the engine will accept full throttle between about 2750 and 3250 rpm.

  3. Full Throttle - upper midrange

    Set the needle height for the best power between about 4000 and 6000rpm. If the engine pulls better when cold but 'soft' when at operating temperature, it is too rich in the midrange and the needle should be lowered.

  4. What if the best position for test 2 is clip #2 and the best position for test 3 is clip #4?

    You probably need the next richer (i.e. steeper taper) needle to change the 4000 to 6000 rpm performance without affecting the other ranges.

    I ended up with (this was in August, 1994):

Initial Results

After the weather turned colder (into the 30s), I started getting a low speed 'stumble' as I rolled the throttle open. I ended up raising the needles to clip position #4. In late Summer, 1995, I started smelling gas when the bike idled, and the spark plugs were jet black. I did the 'Idle Drop' procedure and ended up with the pilot screws out 3/4 turn total from closed. On September 30, 1995, I took the ST to Tri-City Cycle (call Mark Dimmick at 970-667-4771) to verify the mixture by running it on the dynomometer. The low speed exhaust gas was about 2% CO (safe, but barely), and the very high speed (in excess of 7500 rpm) CO was about 6%. Ideally, the CO should be between 3% and 4%. Mark recommended adjusting the pilot screws out about 1/4 turn more and using #138 or even #135 main jets. I will need to talk with him some more about which jets to use.

Incidentally, the rear wheel power output at 5000 feet above sea level is 92hp (this bike has 52,000 miles on it). The output at sea level would be higher: according to Mechanical Engineers' Handbook by Lionel S. Marks (for decades the technical reference for mechanical engineering), engine output is decreased by approximately 15% at 5000 feet compared to sea level. I have seen 142mph on the speedometer while climbing a slight grade at the same 5000 feet above sea level, which is a small improvement over stock.

February 3, 1997

I finally got around to finalizing the jetting on my bike. I installed a set of #136 main jets and adjusted the pilot screws to 1 full turn out (remember that the pilot jets were enlarged to #39).

A Good Place to Start

I believe, then, that is a good combination of mods:


This procedure results in a somewhat stronger midrange (not that it was lacking to begin with) and a much more satisfying rush from about 6000rpm to redline. Your fuel mileage will decrease slightly
(3-5 mpg), depending on the final adjustment of the pilot mixture. I rather like the increased intake noise, especially when riding around town: I can hear it, but no one else really notices. If you get the pilot mixture set a bit richer than stock, the fuel mixture will burn at a lower temperature, perhaps extending the life of top-end engine parts, especially the valves.

I hope that this account (tome, really) might be of some help to others wishing to get a little more performance out of their ST1100s.


FWIW, the day after I had the ST on the dynomometer, the Twisted Shaft, a local motorcycle club, had 'Track Day' at a local road race track. Never having run on a track before, I was pretty apprehensive about the whole affair. It turned out that everyone was very nice and I received many complements both on my ST and on the rapidity at which the ST was circling the track. Considering that the ST1100 outweighed all of the other bikes by 150-300 lbs, it did pretty darn well.

There were several BMW owners there, all of whom prudently removed their saddlebags prior to running on the track. I, on the other hand, left my bags on - much to everyone's amusement. When asked about this, I stated that it was, after all, a touring bike and there wasn't much point in trying to disguise that fact. THIS touring bike, however, is capable of stomping past Ducatis (a 916. OK, he wasn't really trying that hard) coming off of the corners!